I have a fish whose bottom end floats up in the water like there is a helium balloon attached and has been doing that for at least 2 weeks now, longer I think. It seems she has a swim bladder issue but doesn't seem to get worse or better. If your fish is eating and otherwise acting normally it's up to you what you do, it might recover, though your situation sounds more drastic than mine.
You could get a little bottle of clove oil so you are ready to quietly put it to sleep if the time comes anyway. I believe it is the most humane way for fish to die, better than the freezer or the bash on the head.